You create a PCF component and are ready to test. 
What should be your command to start testing and debugging the component in the local test harness?

Answer options:

A.npm run build
B.pac pcf test start
C.npm start
D.pac pcf new test
E.npm t

Correct Answer: C 
The Components are reusable blocks that you can use for Power Apps applications. Power Apps provides out-of-the-box standard controls (components) for creating applications, like HTML Text, Vertical Gallery, or Edit Form. If your Power Platform solution requires more than standard controls, Power Apps also provides tools to create and reuse custom components. Power Apps Component Framework (PCF) helps you to create these components. 
After creating a PCF component, you need to test the new component. You can start the local test harness from your component root directory by running the npm start command (Number 1) in a command window. The command builds the code and starts the harness (Number 2). 

Then a new PCF Test environment browser window opens (Number 1) with the component’s name on the right panel (Number 2) and the running component in a container (Number 3). The right panel provides three sections: Context inputs (Number 4), Data Inputs (Number 5), and Outputs. 
You can interact with the component inside of the container (Number 3) or use the inputs to change the Form Factor, container sizes, property values. If the PCF component needs data for testing, you can load a CSV file with the data. The test environment reads the ControlManifest.Input.xml file for the required types of inputs. You can add any additional inputs in this file if you need them for testing and debugging the component. 

The test harness can help you see the real-time changes in the component’s index.ts or ControlManifest.Input.xml. If you start your test in watch mode: npm start watch, you will see an automatic reflection of the changes in the files on your component. You also can use the debug functionality of your browser. 
All other options are incorrect. 
For more information about the test harness of the PCF components, please visit the below URLs: 

https://docs.microsoft.com/en-us/powerapps/developer/component-framework/debugging-custom-controls
https://docs.microsoft.com/en-us/powerapps/developer/component-framework/debugging-custom-controls#watch-mode-in-test-harness
https://docs.microsoft.com/en-us/powerapps/developer/component-framework/debugging-custom-controls#debug-code-components-using-native-browsers